I would invest that money in advertising. The word of mouth and facebook can really only get you so far, but posters and cards stay put and can be seen by a great many.
But then again, it also depends on what the business is. It is assuming one would not need to invest any money into equipment.
Although, being inspired by Mastpunjabi's comment, investing the money into other currencies would be an interesting way of building your money. For example, getting your $100 put into British pounds. If the exchange rate is $1.67 to the pound when you exchange it, wait until it reaches $1.90 per pounds (and I have seen it do this, many times. I tend to keep track of exchange rates) and exchange it again into dollars, and you've gained only about $13 doing that, but there are other exchange rates which fluctuate a bit more dramatically than that which may serve as better investments.